Unholy is hasty
Unholy's weights are more or less intuitive if you've been following the class changes through the Cataclysm beta. As with all melee specs, you'll want your hit rating up to the usual 8 percent. After that, we still get the most mileage from strength, but coming up very quickly behind that is haste. Thanks to the fact that haste now reduces our rune cooldowns, you'll find it blows away just about any other secondary stat you can get. Critical strike follows swiftly behind, thanks in large part to pets and diseases now taking advantage of it. Mastery and expertise more or less lag behind at dead last.
What these means for reforging is that first, you'll want to make sure your hit is at the usual 8 percent. Hit up to the spell hit cap is somewhat superfluous still, so if you have any hit percentage above 8 percent, it's probably worth it to reforge it away. Haste is your next target. If you can reforge excess hit, expertise or critical strike rating into haste, you should probably do so. Technically, haste does have a "soft cap" when it reaches the point that we're GCD-capped, but said cap will vary from fight to fight and depending on buffs and skill usage -- and it's still pretty useful past the soft cap, anyway. In a similar vein, go ahead and reforge your expertise into haste or crit, depending on what extra stat you gear has.
As far as gemming goes, you probably won't go too badly if you just stick to your straight strength gems, but reforging is useful in that it opens up a new possibility for us, that of simply reforging away stats to work around gem choices. Therefore, what you may wish to do is actually go for your slot bonuses (using Bold Cardinal Ruby for red slots, Fierce Ametrine for yellow, and Etched Dreadstone for the blue slots). If the slot bonuses isn't very helpful, just go straight Bold Cardinal Rubies. If you're past the hit cap after regemming, just reforge some excess hit rating into haste. Of course, before all this, make sure you have the gems to activate your metagem, which should be a Chaotic Skyflare Diamond.
Your weapon counts with frost
Frost's stat weights currently vary based on whether you're dual wielding or using a two-handed weapon. There are some similarities, though. Both weapon types want to keep hit and expertise at their specific soft caps (for special attacks and for dodging, respectively). However, after that, two-handed weapon frost will value haste, because it requires that haste to proc Might of the Frozen Wastes and to have the cooldowns to spend that runic power once it gets it. Just behind haste, it highly values mastery, since it uses so many Frost Strikes. Strength actually ends up behind these "secondary" stats in the long run, they're so essential.
For dual wielding, after hit and expertise, you'll focus on the more traditional strength, then mastery (again, Frost Strike supremacy here), then haste, as dual wielding does have more trouble with hitting the GCD ceiling. Critical strike rating is relatively unimportant for both specs, as Killing Machine will take care of critical strikes quite handily.
For reforging, this means that you first focus on reforging anything you can to get as close to the hit and expertise soft caps as you can. If you're above those caps, start reforging any excess hit or expertise, or any critical strike rating, into mastery or haste (in that order) for dual wielding. You may also wish to reforge any haste into mastery. For two-handed weapons, reforge excess hit and expertise and critical strike rating into haste, or into mastery if haste already exists on the item.
For gemming, frost will follow the same basic philosophy as unholy. Since we have new, useful blue gem choices and the ability to reforge to take care of any excess hit or expertise on gems, you can pretty much gem for gear bonuses with impunity. Dual wield should use the same gems as mentioned above for unholy. Two-handed-weapon users, on the other hand, will want to focus on haste. That means you'll be using haste-focused gems for your slots, namely Fierce Ametrine for red, Quick King's Amber for yellow and Lightning Eye of Zuul for blue. As with unholy, both flavors of frost death knight will want to grab a Chaotic Skyflare Diamond.
Blood tanks like mastery
Blood is now a tank-only tree and also the one that generally should be the simplest to worry about. Stamina continues to be the ultimate stat for blood, especially since Death Strike always heals a minimum of 10 percent of your base health. This makes gemming very easy. Simply keep stuffing your sockets full of Solid Majestic Zircons.
However, if you insist on getting gear bonuses (such as if a bonus provides +9 or +12 stamina), you'll probably be best off using Guardian's Dreadstones or Accurate Dreadstones for red slots, depending on your stat needs, and Regal Eyes of Zul for yellow slots.
For reforging, you'll want to focus on mastery. Mastery is pretty amazing for blood, since it provides a lot of extra absorption. Essentially, if you have hit or expertise past the soft caps, turn it into mastery. If you have haste or critical strike rating, first of all get better gear, but in the meantime, turn it into mastery. Otherwise, if the gear has parry, turn it into mastery. If the gear doesn't have parry, use dodge instead. Get that mastery, in other words.
Of course, in both cases, the supremacy of stamina and mastery can be pre-empted by the need for more threat. Tanks don't absolutely have to be at the 26 expertise dodge cap or at the 8 percent hit cap, but if you're having trouble keeping threat, it can help to get there. Don't be afraid to regem or reforge for hit and expertise if so.
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